100 Neurological Technique 



still there is reason to believe that the embryo is less than 3 mm. 

 in length, simply make small openings in the chorion to allow 

 the free entrance of the fluid. Place the whole in a copious 

 amount of the fixing fluid and change the fluid after 1—2 hours. 



The smaller human embryos are the most difficult to obtain, 

 and so are the most rare and most desired. So far the human 

 embryos on record which have been obtained of lengths less 

 than 2 mm. (less than 10 days) are very few. 



Embryos of more than 5 mm. in length will safely allow 

 both the chorion and the amnion to be removed. The allantois at 

 this stage has become a part of the umbilical cord. In removing 

 the arnnion, leave the umbilical cord attached to the embryo, so 

 that the embryo may be handled by means of it. 



The length of an embryo, roughly speaking, is the length of 

 a straight line passing through its longest axis. This measure- 

 ment, therefore, does not include either of the flexed ends, 

 although in the stages from 3 to 6 weeks considerable of the 

 actual length is represented in the flexures. To measure the 

 smaller embryos it will be safer to lay the embryo on the scale 

 rather than the scale on the embryo. Measurement of the 

 smallest should be postponed till after fixation. 



As to the fixation of the embryos, the following suggestions 

 are offered : 



I. Embryos of 10-25 days or less (2-7 mm. or less) should 

 be placed in 15-20 times their volume of Van Gehuchten's flu-id 

 (A) for 6-24 hours. 



At the end of this time the fluid should be replaced with 80 

 per cent, alcohol. Tightly close the vessel and set aside. In 

 the alcohol the specimen will be preserved indefinitely. 



While Van Gehuchten's fluid does not have so great a pene- 

 trating power as Zenker's fluid (B), it will fix the smaller 

 embryos perfectly well, and is recommended here because it is 

 one of the best of fixing fluids for the study of karyokinesis and 

 general cytology (see Method I). 



2. Embryos of from 4 to 8 weeks (8-25 mm. in length) 

 should be removed entirely from the amnion and fixed in 15-20 



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